The invention relates generally to the formation of photosensitive organosilicon films by plasma-initiated polymerization utilizing direct application of electrical energy, and more particularly, to the formation of photosensitive films from a silicon donor and an organic precursor for use as a selective wetting material.
Selective wetting materials can be used in applications associated with the fabrication of electrical circuits.
An integrated circuit is a three-dimensional structure of alternating patterned layers of conductors, dielectrics and semiconductor films. The patterned layers are formed by lithographic processes that consist of two steps: delineation of the patterns in a radiation sensitive material, usually a polymer; and transfer of the patterns into underlying, previously deposited thin films, using an appropriate etching technique.
Conventional photolithography (that is, photolithography employing light at 350 to 450 nm wavelength) is used to fabricate solid-state devices with features of 6 xcexcm or smaller. Incremental improvements have allowed the use of light in the range of 350 to 450 nm wavelength to produce ever smaller features. However, the ultimate resolution of a printing technique is governed, at the extreme, by the wavelength of the light, or radiation, used to form the image, with shorter wavelengths yielding higher resolution. The same physical principles also govern the resolution limits in microscopy. Additionally, the same basic positive photoresist, consisting of a photoactive compound that belongs to the diazonaphthoquinone chemical family and a novolac resin, has been in pervasive use since the mid-nineteen seventies, and will likely be the resist of choice for several more years. The manufacturing facility has also been driven to cleaner specifications due to the smaller geometries of the parts being produced. The cost of introducing a new photolithographic technology, which includes the cost associated with the development and implementation of new hardware and resist materials, is a strong force pushing refinement of current photolithographic processes and conventional clean room design instead of pursuing revolutionary techniques. The technological alternatives to conventional photolithography are largely the same as they were a decade ago: short wavelengths of light (220 to 280 nm), the so-called deep ultraviolet range photolithography; scanning or projection electron beam; and x-ray or scanning ion beam lithography.
Unfortunately, conventional resists are not appropriate for use with the new lithographic technologies that are necessary for producing line widths under 0.5 xcexcm. The most notable deficiencies of the conventional novolac-quinonediazide resists are their sensitivity and absorption characteristics. Additionally, the absorption of conventional photoresists is too high to allow uniform imaging through practical resist film thicknesses (approximately equal to 1 xcexcm). Thus, no matter which technology becomes dominant after photolithography has reached its resolution limit (that is, 0.3 to 0.5 xcexcm), new resists and processes will be required, necessitating enormous investments in research and process development. The introduction of new resist materials and processes will also require a considerable lead time to bring them to the performance level currently realized by conventional positive photoresists. Clearly, conventional wet chemistry photoresists have limited applicability for the future of solid-state device fabrication. Liquid resist chemicals are very difficult to obtain in a pure state and are expensive. The process of applying a wet resist does not lend itself to large or odd shaped substrates, and the required baking step makes this process unsuitable for temperature sensitive substrates.
Consequently, there are advantages to a photoresist which is easily applied and processed in a vacuum chamber or series of enclosed chambers with no requirement for being brought to atmosphere. Such a photoresist permits the elimination of wet chemistry processes, and consequently, provides substantial benefits in the reduction of costs and generation of hazardous wastes. Furthermore, it eliminates the exposure of operators to the hazardous liquids and vapors generated by the conventional resist process. Finally, the dry plasma deposited photoresist allows the further development of integrated manufacturing modules without the necessity of clean rooms.
Despite intensive research on the plasma deposition of amorphous silicon from monosilane (SiH4 ), there have been only a few reports exploring the formation of Sixe2x80x94Si bonded polymers from monosubstituted organosilanes. Haller reported an example of selective dehydrogenative polymerization, but no photochemical studies were described. See Haller, Journal of the Electrochemical Society A, Vol. 129, 1987, p. 180, and Inagaki and Hirao, Journal of Polymer Science A, Vol. 24, 1986, p. 595. Studies on the plasma chemistry of methylsilane (MeSiH3 ) have involved higher radio-frequency powers and temperatures which promote formation of amorphous silicon carbide (SiC) rather than reactive polymeric product. See Delpancke, Powers, Vandertop and Somorjai, Thin Solid Films, Vol.202, 1991, p. 289. Low power plasma polymerization of tetramethylsilane and related precursors has been proposed to result in the formation of Sixe2x80x94Cxe2x80x94Si linkages. See Wrobel and Wertheimer, Plasma Deposition, Treatment and Etching of Polymers, Academic Press, New York, Chapter 3. Such materials lack sufficient absorption in light above approximately 225 nm wavelength, but have been studied as far ultraviolet (193 nm wavelength) resists by Horn and associates. See Horn, Pang and Rothschild, Journal of Vacuum Science Technology B, Vol. 8, 1991, p. 1493. Polymer chemistry teaches the use of the basic silanes are insignificant as a monomer for polymerization type of polymer. Furthermore, polysiloxanes are differentiated from the basic silanes, and contrasted as being very important in terms of monomers for polymerization. See Stevens, Malcom P., Polymer Chemistry, An Introduction, Addison-Wesley Publishing Co., 1975: p. 334. As used herein, polymerization is the process whereby monomers are bound together forming various length chains but essentially repeating the basic building block of the monomer without significant chemical or physical alteration of the beginning monomer.
Work has been reported on the synthesis of soluble poly-alkylsilyne network polymers ([SiR]n) which exhibit intense ultraviolet absorption (associated with extended Sixe2x80x94Si bonding) and may be photo-oxidatively patterned to give stable siloxane networks. See Bianconi and Weidman, Journal of the American Chemical Society, Vol. 110, 1988, p. 2341. Dry development is accomplished by selective anisotropic removal of unexposed material by chlorine or hydrobromic acid reactive ion etching. See Hornak, Weidman and Kwock, Journal of Applied Physics, Vol. 67,1990, p. 2235, and Horn, Pang and Rothschild, Journal of Vacuum Science Technology B, Vol. 8, 1991, p. 1493. The exposed, oxidized material may be removed by either wet or dry fluorine based chemistry. Kunz and associates have shown that this makes polysilynes particularly effective as 193 nm wavelength photoresists. See Kunz, Bianconi, Horn, Paladugu, Shaver, Smith, and Freed, Proceedings of the Society of Photo-optical and Instrumentation Engineers, Vol.218, 1991, p.1466. The high absorbability and the wavelength limits photo-oxidation to the surface, eliminating reflection, and the pattern is transferred through the remainder of the film during the reactive ion etch (RIE) development. Studies of organosilicon hydride network materials containing reactive Rxe2x80x94Sixe2x80x94H moieties have found that such high silicon compositions as [MeSiH0 5]n exhibit superior photosensitivity and function as single layer photodefinable glass etch masks. See Weidman and Joshi, New Photodefinable Glass Etch Masks for Entirely Dry Photolithography: Plasma Deposited Organosilicon Hydride Polymers, Applied Physics Letters, Vol. 62, No. 4, 1993, p. 372. However, cost and availability of the exotic organosilicon feedstocks have significantly inhibited the transfer of such photosensitive organosilicon hydride network materials into microcircuit fabrication. Further, films deposited from single component organosilicon feedstocks possess limited latitude in alteration of deposited film characteristics, such as the radiation frequency of photosensitivity and selectivity during etch processes.
Organosilicons have been used to produce various types of films, as disclosed by the following references.
Sachdev et al. teaches the deposition of an organosilicon film by plasma polymerizations. However, the deposited film is not itself photosensitive. Sachdev et al. uses the film as a barrier and uses a conventional spun-on resist to pattern it. Sachdev et al. does not teach the use of a plasma-deposited film as a photoresist; a conventional photoresist is used to pattern the plasma-deposited film, which is then used as an etch mask. Endo et al. describes the plasma deposition of a silicon carbide layer. A single organosilicon gas that may react with other materials is used to deposit a durable silicon carbide type of film. Furthermore, Endo et al. does not teach film photosensitivity. Nakayama et al. discloses the use of an oxygen plasma to modify a silicon surface to allow a conventional photoresist to adhere to the material. The conventional photoresist is then used to pattern the silicon layer. A photosensitive organosilicon film is not deposited. There is no plasma depositing of a film; the oxygen plasma is used to modify the surface structure which is followed with conventional techniques for patterning the silicon film. Geisler et al. describes a method of plasma-coating a plastic substrate using organosiloxanes and an inert gas to facilitate adhesion of a reflective layer of a metal with minimal surface damage to the substrate. None of these references teach a selective wetting material formed from a photosensitive plasma polymerized organosilicon film produced from a two-component plasma reaction wherein the first component comprises a silicon donor that is non-carbon containing and non-oxygenated and the second component comprises an organic precursor that is non-silicon containing and non-oxygenated, with selective regions of the film being photo-oxidized to produce a selective wetting material that when brought in contact with a liquid, the liquid will selectively wet and adhere to the photo-oxidized regions of the film, and will not adhere to the non-photo-oxidized regions of the film.
A conventional circuit board is produced by depositing an electrically conductive material, such as silver, on the surface of a substrate (or circuit board) formed from an electrically insulated material, such as a glass epoxy laminate. Selected regions of the deposited electrically conductive material are removed by etching in a patterning process that can be accomplished by photolithography. The remaining regions of electrically conductive material form the printed conductors on the circuit board. The process is inefficient in that the entire surface of the board is first coated with the electrically conductive material, and then some of the deposited material must be removed to define the printed circuit paths or conductors on the board. Therefore there is the need for a process that can form printed circuits on the board without first blanket coating of the board with the electrically conductive material and then selectively removing regions of the electrically conductive material.
One aspect of the present invention is a process for forming a plasma-deposited selective wetting material on a substrate by forming a photosensitive film on a substrate from a two-component plasma reaction. The first component comprises a non-carbon containing and non-oxygenated silicon donor and the second component comprises a non-silicon containing and non-oxygenated organic precursor. The film is patterned with radiated electromagnetic energy through a mask in the presence of oxygen to produce a selective wetting material that has one or more photo-oxidized regions. When the selective wetting material is placed in contact with a liquid, the liquid selectively wets and adheres to the photo-oxidized regions and does not adhere to the non-photo-oxidized regions.
When the liquid is an electrically conductive material, the deposition and drying of the liquid adhering to the photo-oxidized regions can form printed circuits on a circuit board.
When the non-photo-oxidized regions of the selective wetting material are removed and the substrate is an optically transparent material, the substrate with the remaining photo- oxidized regions covered by the deposited and dried liquid can form a photomask.
Additional objects, advantages and other useful features of the invention will become apparent to those skilled in this art from the following description, wherein a preferred embodiment of this invention is shown and described, simply by way of illustration of one of the modes best suited to carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various, obvious aspects, all without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature, and not restrictive. The objects and advantages of the invention may be realized and attained by means of the processes particularly pointed out and claimed in the appended claims.